In recent years, injection foaming products are used as a shock absorbing material and a part with a soft feeling against vibrations and noises in various product field, for example an internal part or an external part of an automobile and the like, a consumer electric product, an information equipment and the like. In particular, a thermoplastic elastomer composition is attracting attention as a material enabling easy molding and easy foaming. Such thermoplastic elastomer composition can be a thermoplastic elastomer composition capable of dynamic crosslinking, and it is known that an injection foaming product can be obtained with a thermoplastic elastomer composition disclosed for example in JP-A-H6-73222.
However, a crosslinking rubber component contained in such dynamically crosslinkable thermoplastic elastomer composition cannot be uniformly foamed and a crystalline polyolefin alone causes a uniform foaming, an inhomogeneous injection foaming product as a whole is obtained. Also since a foaming gas escapes from the surface of the molded article, the surface is not smooth, thus being inferior in appearance. Further odor and discoloration are not reduced sufficiently with the injection foaming product obtained by employing such a thermoplastic elastomer composition. In addition, there are many problems to be solved, such as that the manufacturing process is complicated, that a usable crosslinking agent is expensive, that the application is limited because of a contamination by the crosslinking agent, and the like. And also a non-crosslinked type has a drawback for example of a high compression set, as the obtained injection foaming product does not have a crosslinked structure.
An injection foaming product obtained by employing a thermoplastic elastomer composition described for example in JP-A-H6-73222 as a thermoplastic elastomer composition is flexible in comparison with a prior article. On the other hand, it is also possible to foam a non-crosslinking type olefinic thermoplastic elastomer composition, and such composition can be easily and uniformly foamed by melting.
However, according to the above-mentioned foam injection molding method, since the cell diameter of the injection foaming product becomes large, there is encountered a drawback that an injection foaming product having an adequate flexibility and excellent in cushioning property and the like cannot be obtained. Further the cell diameter is not uniform, and in particular the injection foaming product tends to have significantly different cell diameters in a vicinity of a gate and in an end portion.